In one typical machining operation, a machine tool motor rotatably drives a spindle shaft within a bearing housing, with the motor operatively coupled to one end of the spindle shaft. The opposite end of the spindle shaft extends outside of the bearing housing, and it holds a chuck or other tool-holding device which rotates with the spindle shaft to perform a machining operation on a workpiece. For precision machining operations with critical machining tolerances, the bearing housing and the rotatable spindle shaft must cooperate to precisely rotate the tool-holder about a desired axis, such as vertical or horizontal, over relatively long periods of time. For some applications, such as in the automobile industry, a machining "assembly" line may include as many as three hundred successive machining operations. If one machine tool goes down, for instance, due to machining inaccuracy resulting from problems with the spindle bearings or the spindle itself, it becomes necessary to shut down the entire line at tremendous cost to the manufacturer.
For many machine tools, one area of susceptibility is the seal between the inside of the stationary bearing housing and the rotatable spindle shaft, where the tool-holding end of the spindle shaft extends out of the housing. It is absolutely critical to maintain an effective seal at this joint.
For instance, it is extremely critical to prevent ingress of contaminant materials such as metal shavings or chips from the machined parts, machine tool coolant which is typically sprayed from a nozzle toward the position where the tool contacts the workpiece, and also to prevent the potentially harmful effects generated by humidity, pressure and/or temperature fluctuations. One such effect caused by ingress is liquid condensation. It is common for the coolant to be sprayed continuously at a relatively constant rate, and this results in coolant deflection and splashing on nearby surfaces, including the joint between rotating spindle and the bearing housing. Also, many machining operations require multiple coolant streams to be directed at the spindle, to provide continuous washing of metal chips, i.e., a coolant "chip wash". If ingress of coolant occurs, the coolant is capable of causing severe damage by washing out the lubricant grease for the spindle bearings, which can result in elevated bearing temperatures. In some extreme instances, this can result in catastrophic bearing failure.
Particularly over the past ten to fifteen years, it has become common to use labyrinth-type bearing seals to isolate the inner portions from the outer portions of a spindle shaft of a machine tool. These seals typically include a stator (sometimes referred to as a cap) which is mounted, as by press fitting, into the bearing housing, and which includes radially oriented labyrinth grooves. The labyrinth passage could be formed by the spacing between the stationary and the rotary parts. A rotor fits axially into the stator, revolves with the spindle, and is held in place on the rotating member by static drive rings and/or a tight fit. The labyrinth structure is designed to require multiple changes in fluid flow direction, with accompanying changes in fluid pressure, with the objective of minimizing the possibility of coolant ingress to the bearing. The structure also includes an expulsion port designed to expel any fluid contaminant that may work its way into the seal structure. U.S. Pat. No. 5,378,000 shows one such labyrinth-type bearing seal.
While labyrinth-type bearing seals have proven suitable for some applications, they have also experienced deficiencies in other important applications. One reason for these deficiencies relates to an increase in the performance expectations for bearing seals for machine tool spindles. More specifically, over the past five to ten years there has been an increased awareness of the potential hazards of overexposure of human operators to machine tool coolants and the particles/chips generated by machining. For this reason, and because almost all machine tool coolants are classified as hazardous materials from an environmental standpoint, there has been a movement toward enclosing the machining area of machine tools, usually within some type of movable or closable shroud or enclosure. The shroud reduces exposure of the human operator to potentially hazardous materials such as liquid coolant, machine tool lubricating oil or metal chips produced during machining operations.
Unfortunately, the increased use of such shrouds has produced some unintended adverse consequences. For instance, one noticeable effect of these machine tool shrouds has been the tendency of machine tool builders and/or operators to pay less attention to the amount of coolant necessary for use, since the shroud shields the operator from splashed or oversprayed coolant. This generally results in increased coolant usage, with a corresponding increase in the ingress susceptibility of the bearing seal because of this greater coolant volume. This is also true with respect to the use of the coolant chip wash, which may propel the chips toward the seal.
Also, depending on the particular machining operation, the orientation and/or shape of the shroud may cause an increase in the accumulation of metal chips near the bearing seal. Even though the relatively large metal chips may be too large to work their way past the seal, they may sufficiently interfere with proper operation of the seal so that during use the structure becomes more susceptible to coolant ingress.
Thus, even though a labyrinth-type bearing seal may be suitable for extended use for a particular machine tool operated under conditions prevalent ten years ago, that same bearing seal may not perform sufficiently for the same machine tool under operating conditions prevalent today. It alone can not withstand the increased coolant volume coupled with the increased accumulation of metal chips.
Other bearing seals have been used for spindles, such as rubbing seals which typically include a rubber lip. One advantage of a rubbing seal is the positive circumferential contact along the seal joint. However, rubbing seals have rotational speed limitations, due to excessive heat build up from friction which adversely affects spindle performance.
Some seal configurations have been adapted to accommodate the features of the labyrinth seal and the rubbing seal, with the labyrinth portion located closer to the joint than the rubbing seal. For some of these configurations, during operation, purge fluid from the bearing housing is introduced between the labyrinth seal portion and the rubbing seal portion to prevent ingress of coolant or other potential contaminants. While the purge fluid may improve the effectiveness of the labyrinth seal portion, the labyrinth seal joint still remains open when the purge fluid is turned off, so the labyrinth portion of the seal is still susceptible to liquid ingress. This problem is also true with respect to a labyrinth/mini-maze seal. Moreover, the use of purge fluid in combination with a labyrinth/rubbing seal structure still does not solve the heating problem of the rubbing seal, so there are still speed limitations.
Another bearing seal, disclosed in U.S. Pat. No. 4,565,378, uses a labyrinth in combination with a rotatable contact seal, with compressed gas introduced between the contacting surfaces to lift the seal and form a gas cushion between the surfaces. During low speed operation, the contact seal is relied on to prevent ingress. During high speed operation, the gas cushion is relied on. The success of this seal depends upon centrifugal forces which cause the seal to move out of contact with the opposed contacting surface, and outflow of the compressed gas which forms the gas cushion. However, there does not appear to be any structure for assuring or maintaining uniformity in seal movement or uniformity in fluid outflow around the periphery.
Therefore, there is a need for an improved and more reliable bearing seal for machine tool spindles.